PROJECT SUMMARY It is known that breast cancers with high tumor interstitial pressure (TIP) respond poorly to chemotherapy because of poor drug delivery (i.e., the high pressure prevents the drug from entering the tumorous tissue). Recent data suggests TIP is a potentially useful parameter for assessing whether a particular breast cancer will in fact ever respond to systemic anticancer treatment. Moreover, our group has exciting data that would suggest noninvasive pressure estimation in breast cancer tissue can be performed in real-time using a novel contrast agent and custom technology we have termed TIP estimation using US (TIPE-US). The first aim of this project is to develop a monodisperse phase-change contrast agent (PCCA) that is a stabilized liquid nanodroplet under ambient conditions but can be triggered using US energy to undergo a phase transition into a highly echogenic microbubble (MB). Importantly, this phase transition is sensitively governed, and linearly related to, the surrounding hydrostatic fluid pressure. We will then determine the optimal US parameters (energy) required for activating these PCCA while precisely varying the surrounding fluid pressure across a range of physiologically relevant values. A programmable US scanner will be customized to initiate and detect PCCA activation and then convert the US energy used into a corresponding pressure value via lookup tables. In the second aim, we will evaluate the potential of TIPE-US for monitoring early tumor response (or lack thereof) to anticancer therapy using an animal model of breast cancer. The outcome of the proposed experiments will establish whether TIPE-US is a viable new method for measuring intratumoral pressure in cancerous tissue. This innovative technology may ultimately be utilized to both measure TIP in cancer patients and for monitoring treatment in the neoadjuvant setting to differentiate between responders and non-responders, which in turn will allow for better, individualized treatment options to be selected.